There is no guarantee that python package will work with a different Python version, especially those with C extension.
Indeed strech-slim may be a good idea.
Here is the implementation: https://bugs.tryton.org/issue6876
After 1.10 --link is deprecated . Now is recomended run containers using own defined networks  .
I think we are in the exception to still use --link: https://docs.docker.com/engine/userguide/networking/work-with-networks/#basic-container-networking-example
Indeed it is still the example provided by the postgres image and describing in example how to setup an overlay network seems out of the scope.
Any way, I do not see any problem in using an overlay network with the current image as far as the database hostname is postgres (and even it is possible to setup your own TRYTOND_DATABASE_URI).